Photopolymerizable compositions are materials which undergo changes in the physical properties of the composition as a result of a polymerization reaction induced by light irradiation. Photopolymerizable compositions are widely used in the fields of printing, printed circuit, fine working such as ultra LSI, painting, ink, hologram recording and three dimensional formation, and the application of such compositions is expanding. Photopolymerizable compositions fundamentally comprise an addition-polymerizable, ethylenically unsaturated compound and a photopolymerization initiator. This kind of composition polymerizes upon light irradiation and as a result, it is cured and insolubilized. When a binder resin having film forming ability and a thermal polymerization inhibitor or the like are added to a photopolymerizable composition and formed into a film, a desired cured image can be formed by irradiation of the film with light through a desired negative image and removal of the non-irradiated area by an appropriate solvent (hereinafter referred to simply as "development"). Such an image formation method is well known to be very useful in preparing a printing plate or the like.
Photopolymerization initiators conventionally used in photopolymerizable compositions include benzyl, benzoin ether, Michler's ketone, anthraquinone, acridine, phenazine and benzophenone. However, the composition comprising such a photopolymerization initiator is low in sensitizing rate and extremely poor in photopolymerization ability to visible light at 400 nm or more as compared with the photopolymerization ability to light in the ultraviolet wavelength region of 400 nm or less. Accordingly, the scope of application of a photopolymerizable composition comprising a conventional photopolymerization initiator is strictly limited.
A photosensitive composition which can be highly cured by irradiation with a small amount of light has long been in demand. Such a photosensitive composition is useful as a photosensitive material suitable, for example, for non-contact type projection exposure. Further, in recent years, an image information technique using a visible laser has come into a practical use, and a photosensitive composition having a high photopolymerization ability to visible light is keenly demanded. As a visible laser light, an Ar.sup.+ laser light having a wavelength of 488 nm is promising. Further, energetic investigations have recently been made with a light source having a long wavelength, such as a YAG-SHG laser light having a wavelength of 532 nm and a He-Ne laser light having a wavelength of 543 nm.
With respect to a photopolymerizable composition comprising a photopolymerization initiation system sensitive to visible light, several proposals have been hitherto made. For example, U.S. Pat. No. 2,850,445 has reported that certain kinds of photosensitive dyes such as Rose Bengal, eosine and erythrosine exhibit effective sensitivity to visible light. Also, an improved technique has been reported using a composite initiation system comprising a dye and an amine (see, JP-B-44-20189 (the term "JP-B" as used herein means an "examined Japanese patent publication")), a system comprising a hexaarylbiimidazole, a radical generating agent and a dye (see JP-B-45-37377), a system comprising hexaarylbiimidazole and p-dialkylaminobenzylidene ketone (see JP-B-47-2528, JP-A-54-155292 (the term "JP-A" as used herein means an "unexamined published Japanese patent application")), a system comprising a 3-ketocoumarin compound and an active halogen compound (see JP-A-58-15503) and a system comprising a substituted triazine and a merocyanine dye (see JP-A-54-15102). These techniques are surely effective to visible light but the sensitivity achieved is not high and, in practical viewpoint, fails to reach a sufficiently satisfactory level. JP-A-2-244050 proposes a system comprising a dye having a 4-thiazolidinone skeleton and a radical generating agent as a highly sensitive initiation system. This system is surely highly sensitive, however, the sensitivity is still insufficient.